1. Field of the Invention
The present invention relates to a switch circuit having an excess-current detection function for on/off-control supply of electric power from a power source to a load.
2. Description of the Related Art
(1) Prevention of a short circuit of an electric circuit which causes a failure of an apparatus and a fire has a critical problem which has been investigated to be solved for a long time. However, various states of the short circuits have inhibited a countermeasure capable of appropriately operating against any state of the short circuits and having a simple structure from being realized.
Hitherto, fuses have been most widely employed to protect the equipment and electrical lines from a short circuit. The fuse is provided with a metal conductor portion and arranged in such a manner that the metal conductor portion is melted when the temperature of the metal conductor portion has been raised to a level higher than a predetermined level. In general, the fuse is disposed at an intermediate position of the electric line which must be protected, for example, a power supply line connected to a load in such a manner that the fuse is in series connected to the power source. When an excess current flows in the fuse and thus the temperature of the metal conductor portion is raised to a level higher than the foregoing set level, the metal conductor portion is melted so that supply of electric power through the electric line, which must be protected, is interrupted.
(2) On the other hand, miniaturization of electric products has been attempted by reducing the size of electronic elements arranged to be mounted on the electric product. For example, a switch circuit has been employed in which a switching device for controlling supply of electric power from the power source to a load and a control circuit for controlling the operation of the switching device are integrally mounted on a substrate. As a switch circuit of the foregoing type, there has been employed a circuit having a protective circuit arranged to, in addition to simple on-off control of the switching device, detect generation of abnormality, such as an excess current, excess voltage, overheat and the like, and to control the switching device in response to detection of the abnormality.
The conventional technique (1) is required to design the above-mentioned set value in consideration of deleting with respect to the environment temperatures because the heat accumulation ratio is lowered and melting time is elongated when the environment temperature is low because the metal conductor portion has been melted by accumulated heat.
The short circuits include a state in which an excess current flows continuously and a state in which pulse-like excess currents intermittently flow. In the latter case, the heat accumulation ratio is reduced as compared with the former case. Therefore, there arises apprehension that the melting time is elongated or the fuse cannot be melted though an excess current is flowing. Thus, devices and electric lines cannot reliably be protected by using the fuse from intermittent pulse-like excess currents.
The switch circuit in the conventional technique (2) is arranged to detect an excess current in such a manner that the switching device is immediately switched off when an electric current having a level considerably higher than a rated current for the circuit or the load has flowed so as to self-protect the switch circuit. That is, the foregoing technique is not structured to protect the electric line and the load from an excess current having a level somewhat higher than that of the rated current set for the electric-line elements between the power source and the load or that for the load.